The present invention relates to apparatus for heat sealing thermoplastic sheeting and, more particularly, to such apparatus of the so-called L-sealer type adapted for use in packaging articles in heat-shrinkable plastic film.
In recent years, it has been increasingly popular and commonplace to package many commercial articles in close-fitting transparent plastic film. This packaging is accomplished conventionally by wrapping the article to be packaged loosely in a heat shrinkable plastic film, sealing together the abutting edges of the film to enclose the article, and subjecting the package to sufficient heat to activate shrinkage of the film tightly about the article.
Machines commonly referred to as L-sealers are available for commercially accomplishing the preliminary wrapping procedure as above-described. Heat shrinkable plastic film in continuous sheets pre-folded along a central longitudinal edge and packaged in convenient rolled form is utilized for this purpose. L-sealing machines characteristically provide a sealing table for supporting the plastic film with the article inserted between the folds of the film and a pair of L-shaped jaws at the table adapted for receiving and grippingly engaging the film therebetween. One of the jaws is provided with a heated wire sealing arrangement to simultaneously sever and seal the film along a continuous L-shaped seal line extending parallel to the folded edge of the film and transversely between the parallel seal extent and the folded edge. In continuous operation, the transverse seal formed in making each package provides the initial transverse seal for the next package so that each described L-sealing operation provides complete package formation. Representative examples of L-sealing equipment of the described type are found in U.S. Pat. Nos. 3,347,729; 3,490,981; Re. 30,010 and 4,035,983. A more sophisticated L-sealing apparatus adapted for automatic high speed operation is disclosed in U.S. Pat. No. 4,219,988.
As will be appreciated, it is extremely important in the described L-sealing operation that a strong seal be formed uniformly and continuously along the L-seal line in order to insure proper shrinkage of the film about the package. The satisfactory accomplishment of this purpose has been an ongoing problem in the L-sealing industry for many years. Virtually all L-sealers utilize as one jaw a sealing bed fixed to the machine frame at the sealing table surface and as the other jaw a moveable sealing arm pivoted to the frame and carrying the heated L-seal wires. Formation of the L-seal is a function of two basic criteria: (1) the proper alignment of the sealing bed and the sealing arm with one another, and (2) the exertion of uniform sealing pressure along the entire L-seal line when the sealing bed and arm are closed together for sealing operation.
To facilitate proper alignment of the sealing bed and arm, substantially all L-sealers provide adjustable mounting of the pivots for the sealing arm and adjustable support bolts for the sealing bed to permit the selective adjustment of their relative positions with respect to the machine frame to achieve proper desired alignment. However, it is common for the sealing bed and sealing arm to become misaligned during use and therefore realigning adjustment is constantly required. Alignment using the described adjustments is time-consuming, tedious and expensive, both in terms of labor and machine downtime, and accordingly is widely viewed as perhaps the greatest disadvantage of conventional L-sealing machines.
Furthermore, even with a properly aligned sealing bed and sealing arm, problems may still be encountered in conventional L-sealers in obtaining properly uniform surface contact and pressure between the sealing bed and arm along the entire extent of the L-seal line. Some L-seal apparatus are adapted for manual movement of the sealing arm into engagement with the sealing bed, whereby the achievement of uniform sealing contact and pressure depends greatly upon the skill and consistency of the operator. More often, L-sealing apparatus is provided with a mechanical or pneumatic latching or similar arrangement to exert force on the sealing arm to press it into engagement with the sealing bed. Some apparatus of this type experience problems with the warping or deformation of the sealing arm which prevents uniform contact between the sealing arm and bed even when they are initially properly aligned. Typically, this problem is solve by constructing the sealing arm to be sufficiently rigid and sturdy to avoid such distortion. In the above-identified U.S. Pat. No. 3,490,981; Re. 30,010; and 4,035,983, L-sealing equipment is provided with an L-shaped arm pivoted at its free ends to the frame about co-linear pivot axes with a latching arrangement being provided at the corner of the arm, in order to avoid misaligning distortion of the sealing arm when the latching mechanism is activated.
L-sealers, because of the exposed heating wires and the significant pressures exerted between the sealing arm and bed, also require the use of safety mechanisms to avoid accidental injury to the operator or damage to the machine or articles to be packaged resulting from closing of the sealing arm and bed onto the operator or another obstruction. Typically, such safety mechanisms, although effective for their intended purposes, have been unnecessarily complicated and sophisticated and hence additionally serve to increase the cost of manufacture as well as the cost and frequency of repair and maintenance.
In contrast, the present invention provides an improved L-sealer uniquely adapted to automatically provide substantially continuous and uniform surface contact and pressure along the entire L-seal line substantially regardless of any misalignment of the sealing arm and bed and without requiring fine adjustment thereof. The present invention also provides a simple and effective safety mechanism for de-actuating operation of the apparatus when an obstruction is encountered by the sealing arm.